4.7ns is more like 213MHz. 74 series logic is routinely used in circuits operating at 10+MHz, so I'm sure you'll be fine. If you're only level converting the MIDI signal, even a resistor divider would probably do just fine.

A resistor divider would work fine for conversion of 5V to 3.3V as long as your input Z is high and output Z is low (compared to your network Z). Buffered logic is insensitive to impedance. Essentially, choose the tool you require.

Yes, they do exist. But they are few in actual numbers and even if you happen to have one it's always in General MIDI, not LA Synth. Quite understandable since LA synth was pretty much old news when PCMCIA cards came available and GMidi was The Thing. You can easily emulate MIDI stuff with modern laptops through DosBox+Munt. Maybe even go with USB MIDI interface and real MIDI modules. But only plausible way to do MIDI with any true hardware solution in genuine retro laptops is through those PCMCIA sound cards. And good ol' LA synth in them is nonexistent.

Ah, but I digress. I just wanted to express my fondness for genuine hardware laptop MIDI solutions. I understand that this daughtercard project is it's own thing.

I've mucked around with a MeeGoPad Compute Stick with the older Z3736F Atom and it ran DOSBox games as well as Munt without any issues. I can see similar tablets selling for around $80 to $100, so I think this is the direction I'm leaning towards. All we would need is a MT-32 themed tablet stand

gdjacobs wrote:A resistor divider would work fine for conversion of 5V to 3.3V as long as your input Z is high and output Z is low (compared to your network Z). Buffered logic is insensitive to impedance. Essentially, choose the tool you require.

It's true, for 5v -> 3.3v a resistor network would be fine; but I need to go 3.3v -> 5v as well, so I think I will need at least one active chip to perform that task, and if I've got one chip to go one direction I might as well have an active chip to go the other direction. The buffer chips are pretty cheap anyway, like $0.65... almost cheaper than resistors.

I had to think about this a lot over the weekend, thinking that I would throw out the whole idea of a WaveBlaster card and just go with the standalone unit and do the whole thing in 3.3v logic, but I think I came up with a good design, I think better than the first revision, so I'm happy; just means going back to the drawing board and entering t he new design into KiCAD again...

RJDog wrote:It's true, for 5v -> 3.3v a resistor network would be fine; but I need to go 3.3v -> 5v as well, so I think I will need at least one active chip to perform that task, and if I've got one chip to go one direction I might as well have an active chip to go the other direction. The buffer chips are pretty cheap anyway, like $0.65... almost cheaper than resistors.

Well, I'm just being pedantic at this point, but a 3.3V '1' is still a '1' in a 5V system (well into the '1' range in TTL logic, and just above the cutoff but still within the range of a 1 for a CMOS device)...

That said, yeah a buffer isn't a huge expense anyway and I was mainly just illustrating the point that at these speeds you don't need anything fancy.

EDIT: Ok, I take that back about CMOS logic. The spec apparently says 3.5V minimum, but the datasheet for e.g. a 74HC125 says 3.15V so YMMV

RJDog wrote:It's true, for 5v -> 3.3v a resistor network would be fine; but I need to go 3.3v -> 5v as well, so I think I will need at least one active chip to perform that task, and if I've got one chip to go one direction I might as well have an active chip to go the other direction. The buffer chips are pretty cheap anyway, like $0.65... almost cheaper than resistors.

Well, I'm just being pedantic at this point, but a 3.3V '1' is still a '1' in a 5V system (well into the '1' range in TTL logic, and just above the cutoff but still within the range of a 1 for a CMOS device)...

That said, yeah a buffer isn't a huge expense anyway and I was mainly just illustrating the point that at these speeds you don't need anything fancy.

EDIT: Ok, I take that back about CMOS logic. The spec apparently says 3.5V minimum, but the datasheet for e.g. a 74HC125 says 3.15V so YMMV

The low threshold of a 3.3V TTL high signal is in an indeterminate range for 5V TTL. Lots of situations where that could get you in trouble. If you know you're always getting 3.2-3.3V into the gate, you're probably okay, but buffering eliminates those sorts of concerns.

So, I feel like I'm resurrecting an old thread here, but really I have been quite actively working on this one

So I have what is now Revision 3 of the wavetable board. Admittedly, I am starting to become less convinced that a wavetable card is really the way to go, as it might not be practical for most users as opposed to a standalone-only device. This standalone "host" is obviously designed to accept the wavetable card and turn it into standalone unit, but it is also designed to be able to accept other "generic" wavetable cards and turn them into a standalone unit... yes, very similar to dreamblaster's WaveBlaster Module MIDI Interface Board, but from a "generic" perspective this board also has midi out and thru, and has a USB MIDI interface (auto switches the input to the wavetable card from the MIDI 1.0 ports to the USB MIDI when USB is plugged in). That said, my testing will be limited to ensuring the "MB-32" wavetable card works with it.

I changed to surface mount components for the ICs... mostly because it seemed to be practical to get Atmel microcontrollers with the features and pin count that I want that only come in surface mount packages, so I said screw it, and chose surface mount packages for the other ICs as well.

I'm pretty committed to this design now, and actually have all of the components ordered and received to build it... just waiting on the PCBs now...

Also, any information on the actual output quality of the Orange Pi? The Raspis have a notoriously bad codec driving the headphone jack, so an external audio device (either on an expansion plate or via USB) is virtually a must.

Rackatack wrote:So why not a display that fits in the tower (like a DVD or floppy drive) with something like this?

On the MB-32 wavetable card the "host" connector is intentionally designed to be dual purpose; to not only provide the required connections to the MB-33 host (the standalone enclosure with all I/O, LCD and front panel), but it can also provide a connection to a LCD panel. I designed it that way on purpose so that it could be adapted to drive an LCD when used internally as a WaveTable card with the LCD in, say, a 5 1/4" drive bay. I am designing the board to do this, but don't intend to have the PCB made in this prototyping phase.

gdjacobs wrote:Also, any information on the actual output quality of the Orange Pi? The Raspis have a notoriously bad codec driving the headphone jack, so an external audio device (either on an expansion plate or via USB) is virtually a must.

It is supposed to be 24-bit DAC, and a respectable 96 dB SNR, notably better than the Raspberry Pi which don't really have a DAC, just a reasonably filtered PWM ouput. This is one of the things that attracted me to the OPi. I (unfortunately) only just received the Orange Pi Zero this past week (yay shipping from China) and havent had a chance to really test it yet.